Wood pulp is normally cleaned by letting the washing liquid pass the pulp in counter-current flow, in other words so that the pulp fibres meet progressively cleaner washing liquid as it progresses through the washing device, in order to obtain the best possible washing effect. This is the basic principle in connection with both separate washing devices and continuous washing devices, integrated in a continuous cooking device. This latter technique is described e g in "Continuous Pulping Processes" by Sven Tydholm, pp 173-178.
When the washing device is separate, counter-current flow has no draw-backs, but there are certain specific problems in connection with counter-current flow when the washing is carried out as a final operation in a continuous pulp digester. Thus, the upward flow of the washing liquid slows down the fiber mass and consequently the downward feed of the chip column above. When the production rate and/or the dilution rate is great, sometimes the chip column does not flow downward smoothly under the influence of gravity but jams. Hence, counter-current washing may cut the maximum capacity of the integrated device by 20-30%, which must be considered a serious drawback of counter-current washing. As a consequence, the normal maximum production rate is not greater than 25-30 tons per day per square meter in a continuous pulp digester with counter-current washing. Otherwise the chip column jams, the production rate becomes uneven, and consequently the product quality is lowered, the yield decreases and the washing becomes less efficient.